Exemplary embodiments of the present invention relates to a method for alloying calcium to an aluminum-scandium alloy and an aluminum-scandium-calcium alloy.
Aluminum is preferentially used as a construction material because of its low density; that is, in applications where low mass is desired, such as in transportation vehicles, especially in air and space travel. Aluminum is a light metal and is therefore interesting for such applications, but has the disadvantage that it is relatively soft and has a tensile strength in an annealed state of only 30-50 MPa. The strength values of aluminum can be increased within wide limits by alloying with other metals, and other properties can also be thereby influenced. This is advantageous for lightweight construction, as construction materials having a high specific strength are required here. For example, by alloying scandium in connection with a sufficiently rapid cooling after casting, a much greater strength increase can be achieved through precipitation hardening via a fully or partially coherent Al3Sc phase and/or via dispersoid hardening—that is, if the Al3Sc phases become increasingly incoherent due to excess aging—in addition to increased strength due to mixed crystal formation. Since the density of scandium, at 2.98 g/cm3, is greater than that of aluminum, at 2.7 g/cm3, however, scandium increases the material density and thus also the overall weight.
Aluminum-scandium alloys are well known and their properties are described in the following publications which form part of this disclosure by reference:
A. J. Bosch, R. Senden, W. Entelmann, M. Knüwer, F. Palm “Scalmalloy®: A unique high strength and corrosion insensitive AlMgScZr material concept,” Proceedings of the 11th International Conference on Aluminium Alloys
F. Palm, P. Vermeer, W. von Bestenbostel, D. Isheim, R. Schneider “Metallurgical peculiarities in hyper-eutectic AlSc and AlMgSc engineering materials prepared by rapid solidification processing,” Proceedings of the 11th International Conference on Aluminium Alloys.
In order to reduce the density of said aluminum-scandium alloys, in addition to the adding of magnesium (density 1.74 g/cm3) described in the aforementioned publications, it is particularly possible to add lithium to the alloy, which has a density of 0.5 g/cm3.
The production of aluminum-scandium-lithium alloys is problematic in production, however, since the melt must be handled under protection gas, such as argon. Furthermore, channels and melting pots must be specially lined, such as with CeO, ZrO or other protective metal oxides. The melt reacts easily to air with fire or explosion and has consequently frequently been separated from the environment in past production processes by a protective slag, as well.
U.S. Pat. No. 5,211,910 describes an aluminum alloy that can have scandium and/or calcium at a ratio of 0.5 to 4 wgt.-%.
PCT Publication No. WO 2007/102988 A2 discloses an aluminum alloy that can have calcium and/or scandium in a range from 0.01 to 6%.
In the German Wikipedia, a method is described under the term “Schmelzschleudern” (“melt spinning”) wherein melts, particularly metal melts, are cooled—that is, quenched—at very high velocities
KBM AFFILIPS Master Alloys offers aluminum master alloys on its website, such as aluminum-magnesium alloys, aluminum-scandium alloys or aluminum-calcium alloys.
Exemplary embodiments of the present invention provide a simple and safe method for producing an aluminum-scandium alloy having reduced density.
A method for alloying calcium to an aluminum-scandium alloy in order to produce an aluminum-scandium-calcium alloy has the following steps:
a) combining aluminum, scandium and calcium together in a melt; and
b) quenching the common melt.
Calcium, having a density of 1.55 g/cm3, has a significantly lower volume weight than aluminum, and thus contributes to a reduction of the total density of the alloy when added to an aluminum-scandium alloy.
A material produced from such an alloy is light and still extensively has the strength properties of the aluminum-scandium alloy.
The melt having calcium can be handled under atmospheric conditions without trouble, so that safety precautions such as the lining of channels and pots with oxides and the use of protection gas are not necessary.
The solubility of calcium in aluminum is very low, so that no significant alloy volumes greater than 0.5 wgt.-% have been producible to date. If the melt that fundamentally comprises the alloy partners is rapidly quenched and a rapid solidification process is thus carried out, however, calcium remains extensively in solution in the solid phase.
An aluminum alloy having high strength and low density can thus be produced in a simple and safe method.
Calcium is preferably added to the alloy at a ratio of more than 0.5 wgt.-%. Calcium is thus present in the alloy at a significant ratio and considerably reduces the weight of the alloy and also of the materials produced therefrom.
Calcium is preferably added to the alloy at a ratio which achieves a density less than 2.6 g/cm3. The weight of the alloy can thus be reduced by approximately 5% compared to the aluminum-scandium alloy.
The common melt is advantageously quenched by means of a rapid solidification process at a velocity of more than 11 K/s, particularly 10,000 K/s to 10,000,000 K/s. In a normal metallurgical production path, wherein after the smelting a cast-solidification having slow cooling conditions occurs, it is difficult to add calcium in significant volume to an aluminum-scandium alloy. An Al2Ca phase forms immediately, which is eliminated and the alloy embrittles. If a rapid solidification process is carried out, however, the problem of limited solubility and unintentional early, grossly property-deteriorating elimination of calcium in aluminum alloys can be overcome and calcium remains extensively in solution, since the natural crystallization is prevented by the rapid cooling. The atoms are thus robbed of movement before they can take on a crystalline arrangement and Al2Ca can thus be formed.
Methods suitable for this are all solidification methods in which heat is rapidly extracted from the melt, such as melt spinning, powder atomizing by means of gas or in water, thin strip casting or spray compacting, but also methods wherein a melt is produced in a short period of time and immediately solidifies again, such as welding processes for connecting, surface modifying or generative production of three-dimensional components—so-called “additive manufacturing.”
According to the invention, the common melt is advantageously sprayed onto a substrate as a nozzle jet, wherein the substrate is cooled and rotated during the applying of the common melt. The substrate can be, for example, a copper wheel cooled by water. A temperature difference between the common melt and the substrate results due to the cooling, so that a temperature transfer occurs from the melt to the substrate. The higher the temperature difference, the faster the temperature on the substrate is transferred and discharged by the cooling. The cooling rate, and thus the presence of a rapid solidification in order to prevent the Al2Ca phase formation, furthermore depends on the velocity at which the melt meets the substrate and on the rotation velocity of the rotated substrate.
If the substrate is preferably rotated so quickly that the quenched common melt is spun off from the substrate starting from a point of impact of the nozzle jet on the substrate, the substrate is automatically freed from the solid alloy, already formed by quenching, and is available for subsequently sprayed common melt for cooling. An accumulation of alloy material on the substrate, which is contrary to a rapid temperature transfer from the common melt to the substrate, is thus advantageously prevented. The spun-off common melt forms a band that can be further processed in subsequent method steps.
For example, the band is first chopped small, processed into granulate or powder and then compacted in a pressing and outgassing/baking method into bolts. The bolts—that is, the particulate pre-material—can then be extruded into extruded sections having various cross-sections.
The method is preferably carried out under atmospheric conditions, particularly in contact with air. Measures for protecting the common melt from the atmosphere are no longer required and thus the use of protection gas, vacuum conditions, guard devices and the like can be omitted. This simplifies the method and makes it significantly more cost-effective in comparison to adding lithium to the alloy.
Step a) particularly preferably comprises the step of combining an aluminum-magnesium master alloy into the melt. Magnesium has a density of 1.74 g/cm3. It controls and simultaneously reduces the density of the corresponding alloy. The more magnesium is found in the alloy, the lower the density. The adding of magnesium to the aluminum alloy is sensible up to a ratio of 10 wgt.-%. As a result of the similar melting points of aluminum and magnesium, production of an aluminum-magnesium master alloy is particularly simple to execute.
Aluminum-scandium alloy is a generic term for all alloys that comprise aluminum and scandium. These alloys include all compositions having the formula AlScM1M2M3M4, wherein M1 is a metal selected from the group comprising copper, magnesium, manganese, silicon, iron, beryllium, lithium, chromium, zinc, silver, vanadium, nickel, cobalt and molybdenum, and wherein M2 is a metal selected from the group comprising copper, magnesium, manganese, silicon, iron, beryllium, lithium, chromium, zinc, silver, vanadium, nickel, cobalt and molybdenum.
M3 comprises the group of elements having a certain compatibility with the Al3Sc phase—that is, metal-physical similarity (interchangeability)—and therefore can form the tertiary phase Al3Sc1-xM3x. These are primarily zirconium, niobium, tantalum and titanium.
M4 comprises the group of so-called rare earth metals (element numbers 39 and 57 to 71), which generally have great similarity to scandium. Sc is consequently often incorrectly attributed to the rare earth metals. They can also be added to the alloy to a significant extent, in addition to the scandium, and then form a hardening phase, in addition to the mixed crystal hardening, alone or together with scandium, having comparable stoichiometry to Al3Sc1-xM3x.
Additionally, an aluminum-scandium pre-alloy is preferably introduced to the melt in step a). Scandium has a significantly higher melting point than aluminum and a long holding time must be maintained as a result in order to form an alloy. Because this is expensive it is advantageous when, instead of the pure elements, a pre-alloy is used wherein the scandium is already “melted in” and consequently a shorter holding time must be maintained to form the aluminum-scandium-calcium alloy.
Additionally, an aluminum-calcium pre-alloy is preferably introduced to the melt in step a). Calcium also has a significantly higher melting point (842° C.) than aluminum and the required melting point and thus the holding time are preferably reduced by the pre-alloying.
An aluminum-scandium-calcium alloy has a calcium ratio of more than 0.5 wgt.-%. The density of the aluminum-scandium alloy can thus be reduced in that an easily available and simple to handle metal is comprised in the alloy as an alloy component.
The alloy preferably has 0.2 wgt.-% to 3 wgt.-%, preferably 0.4 wgt.-% to 1.5 wgt.-% scandium. If scandium is comprised in the alloy in the specified volumes, it increases the strength of the alloy but does not so heavily contribute to an increase in density of the alloy that a material produced therefrom would be too heavy for lightweight construction. Alternatively, ytterbium can also be added to the alloy in the cited ratios instead of scandium. Ytterbium is more cost-effectively obtainable than scandium, but has the disadvantage that it improves the strength of the alloy less than scandium.
The alloy preferably has 0.1 wgt.-% to 1.5 wgt.-%, more preferably 0.2 wgt.-% to 0.75 wgt.-% zirconium. Zirconium, in such a ratio in the alloy (Zr/Sc ratio approximately ½ to approximately ¼) makes temperature-supported further processing of the alloy easier and stabilizes it thermally; that is, it reduces the inclination toward “aging,” which is synonymous with an unintended coarsening of the hardening phase Al3Sc by forming an Al3ScZr phase.
Additionally, the alloy preferably comprises 1.0 wgt-% to 8.0 wgt.-%, more preferably 2.5 wgt.-% to 6.0 wgt.-% magnesium. Magnesium reduces the density of an aluminum alloy. The adding of magnesium to aluminum is only sensible up to certain volumes, however, since otherwise negative properties such as brittleness and corrosion sensitivity heavily increase. For that reason, magnesium is preferably comprised in the alloy at the cited ratios.
Furthermore, the alloy also preferably has additional admixtures, also in multiple form, the elements cited in M1, M2, M3 and M4 having the ratios 0.2 to 2.0 wgt.-% which improve the mechanical, physical or chemical properties of the alloy. The presence of undesirable contaminants of a metallic but also non-metallic nature, such as oxides, nitrides, released gases, et cetera in negligible volumes—that is, totaling less than 0.5 wgt.-%—is unavoidable.
The alloy preferably has a density of less than 2.6 g/cm3. The alloy is thus particularly well suited as a raw material for lightweight construction.
In the preferred design, the alloy essentially has the same strength and essentially has the same elasticity module as the pure aluminum-scandium alloy which comprises no added calcium. The alloy thus has the positive properties of the aluminum-scandium master alloy; that is, essentially the same strength and the same elasticity module, but is density-reduced by the presence of calcium and is thus lighter.
An aluminum-scandium-calcium material has more than 0.5 wgt.-% calcium. Such a material is characterized by particularly favorable strength values and a high elasticity module, but has reduced density and is thus particularly well suited for lightweight construction.